README.md

9. Palindrome Number

Determine whether an integer is a palindrome. An integer is a palindrome when it reads the same backward as forward.

Example 1:

Input: 121
Output: true

Example 2:

Input: -121
Output: false
Explanation: From left to right, it reads -121. From right to left, it becomes 121-. Therefore it is not a
palindrome.

Example 3:

Input: 10
Output: false
Explanation: Reads 01 from right to left. Therefore it is not a palindrome.

Follow up:

Coud you solve it without converting the integer to a string?

Solutions (Python)

1. Two Points

class Solution:
    def isPalindrome(self, x: int) -> bool:
        if x < 0:
            return False
        n = 0
        while x // (10 ** n) != 0:
            n += 1
        n -= 1
        while x != 0:
            head = x // (10 ** n)
            tail = x % 10
            if head == tail:
                x %= 10 ** n
                x //= 10
                n -= 2
            else:
                return False
        return True

2. Revert Left Half

class Solution:
    def isPalindrome(self, x: int) -> bool:
        if x < 0:
            return False
        n = 0
        while x // (10 ** n) != 0:
            n += 1
        if n % 2 == 1:
            left = x // (10 ** (n // 2 + 1))
        else:
            left = x // (10 ** (n // 2))
        right = x % (10 ** (n // 2))
        rev = 0
        while left != 0:
            rev *= 10
            rev += left % 10
            left //= 10
        return rev == right

3. Revert Right Half

class Solution:
    def isPalindrome(self, x: int) -> bool:
        if x < 0 or ((x % 10) == 0 and x != 0):
            return False
        rev = 0
        while x > rev:
            rev *= 10
            rev += x % 10
            x //= 10
        return x == rev or x == (rev // 10)

Solutions (C)

1. Elementary Math

bool isPalindrome(int x){
    int origin = x;
    long sum = 0;
    while(x > 0)
    {
        sum = sum *10 + x % 10;
        x /= 10;
    }    
    if(sum == origin)
    {
        return true;
    }
    return false;
}